A. Field of the Invention
This invention relates to the field of ophthalmology and in particular to methods and cameras used to acquire images of the interior of the eye and associated anatomical structures.
B. Description of Related Art
The term xe2x80x9cocular fundusxe2x80x9d refers to the interior part of the eye, opposite the pupil, where structures including the neurosensory retina, retinal blood vessels, optic nerve, retinal pigment epithelium, and other anatomical structures are located. The appearance of the ocular fundus is affected by a wide variety of pathologies, both ocular and systemic, including diabetic retinopathy, age-related macular degeneration, glaucoma, diabetes, hypertension, arteriosclerosis, and many others. Consequently, most routine physical examinations and virtually all ocular examinations include careful examination of the ocular fundus.
One particular eye disease, diabetic retinopathy, is the leading cause of blindness and partial vision loss in this country. The disease affects patients with diabetes, of which there are some 17 million in the United States, with 800,000 new cases diagnosed each year in the United States alone. Diabetic retinopathy is characterized by specific lesions visible during an examination of the ocular fundus including, microaneurysms, dot blot hemorrhages, venous beading, intra-retinal microvascular abnormalities, neovasculraization, vitreous hemorrhage, and others. Fortunately, if detected and treated early, almost all of the vision loss from diabetic retinopathy is preventable. Hence, early and regular ocular evaluation of patients with diabetes are keys to successful outcomes in this patient population.
Routine examination of the ocular fundus is performed using an ophthalmoscope, a small, hand-held device that shines light through the patient""s pupil to illuminate the ocular fundus. The light reflected from the patient""s fundus enters the examiner""s eye, properly focused, so that the examiner can see the fundus structures. Retinal drawings are typically created by hand showing the location of specific lesions. It is now becoming a more common practice to acquire a permanent record of the appearance of the ocular fundus in the form of a set of digital photographs of various regions of the ocular fundus. The photographs provide a more permanent record of the patient""s condition and can be shared among general practitioners and ophthalmologists. They can also be transmitted over networks, enabling remote reading of the images of the ocular fundus by trained readers.
One motivation for acquiring digital photographs of the ocular fundus is the so-called Early Treatment Diabetic Retinopathy Study ETDRS method for diabetic retinopathy staging promulgated by the National Institutes of Health. The ETDRS method calls for photographically acquiring seven, thirty degree, color stereo pair images of the ocular fundus, centralized reading of the photographs, and standardized scoring. The ETDRS standard is an accurate, objective, quantifiable, and reproducible approach to staging this disease, and is considered the xe2x80x9cgold standard by which all other methods are judged.
FIG. 1 is an illustration of the seven fields of the ocular fundus that are photographed in an ETDRS diabetic retinopathy evaluation. The seven fields 1-7 are each 30 degrees in size. Field 1 is centered about the optic nerve ON. Retinal blood vessels V are present in each of the seven fields. The vessels V are found in the locations shown relative to the optic nerve in the human population. The spatial location of the vessels V and their position relative to the optic nerve ON, and the appearance and location of the optic nerve ON in the fundus, is grossly the same in the human population.
Cameras for taking pictures of the fundus are known in the art. See e.g., U.S. Pat. No. 6,296,358 and U.S. Pat. No. 4,715,703, the contents of which are incorporated by reference herein. Typically, in such cameras the operator positions the fundus camera at the correct distance from the eye, and such that it is oriented precisely in the vertical and horizontal directions in such a way that the camera""s illuminating light rays properly enter the pupil of the patient""s eye. A visual target is provided for the patient to look at so that the desired region of the fundus will be imaged. The camera sequentially activates various spatially arranged light sources or targets that the patient looks at to thereby shift their line of sight relative to the optical axis of the instrument. In this manner, the region of interest in the interior of the eye is placed in the optical axis of the instrument and can be captured by the imager in the instrument.
In a first aspect, a method is provided for imaging the interior of the eye of a patient. The method includes the steps of collecting a series of images of the interior of the eye with a camera and providing visual or audible feedback to the patient to shift their line of sight in order to image additional regions of the interior of the eye. The feedback provided to the patient is derived from processing the series of images already acquired to determine regions of the interior of the eye that have been imaged in the series of images. In response to the patient""s reaction to the feedback, the method continues by acquiring additional images of the interior of the eye to thereby acquire additional images of the interior of the eye not previously obtained in the series of images. Thus, the method uses the processing of the images already acquired by the camera in near real time as the mechanism for providing the basis for visual or audible patient feedback to direct the acquisition of additional images of the eye.
The processing of the images will typically require some predetermined instructions or knowledge of what regions of the eye need to be imaged (e.g., the 7 fields of an ETDRS 7 field diabetic retinopathy evaluation, or, as another example, the area extending 40 degrees in all directions from the center of the optic nerve). The instructions for what regions of the eye to image are input into the camera""s central processing unit in any suitable form, such as the 7 fields mentioned previously, a circle of particular size in degrees centered about the optic nerve, or any other suitable form. The processing of the images also may require some a priori knowledge of a structure in the eye that can serve as a base for determining which locations in the eye the camera is imaging. Since the optic nerve has in some gross sense the same shape and size characteristics in virtually all humans, the optic nerve is a suitable candidate. If the optic nerve is used, for example, a pattern representing the optic nerve is stored in the computer. Pattern recognition software is used to compare an image or portion thereof acquired by the camera to the stored pattern representing the optic nerve and to therefore confirm that a given exposure of the camera captures the optic nerve or some component part thereof (in which case additional images would need to be generated to image the entire optic nerve).
Once the orientation of the camera relative to a known structure in the eye, e.g., the optic nerve, is determined, the camera can proceed to acquire a set of images covering the regions to be acquired in the imaging session. Each image acquired by the camera will typically overlap with the previous image by some amount (e.g., 5 degrees) so as to maintain some spatial knowledge of what region of the fundus is in the camera""s current field of view. The computer or central processing system for the camera executes software that continually compares the acquired imagery to the regions of the eye to be imaged in the imaging session. If gaps in coverage are present, the patient can be provided with additional feedback to shift their line of sight in a manner such that regions that have not been imaged (such as any gaps) are subsequently imaged by the camera.
The method and camera of this invention allows for cameras with a relatively narrow field of view to be used. Such cameras typically have higher quality lenses and other optical components, resulting in improved quality of the acquired images. Improved image quality can result in increased spatial resolution and the ability to detect and analyze finer structures in the eye. Cameras having narrower fields of view also are less expensive. Additionally, the method allows for the images to be captured at a high frame rate reducing the total time to acquire a set of images of the ocular fundus.
In one possible embodiment, the feedback provided to the patient comprises audible instructions to the patient directing the patient to shift their line of sight in a direction derived from the processing of the images. Alternatively, the feedback can comprise an illumination source visible to the patient directing the patient to shift their line of sight in a direction derived from the processing of the images. The illumination source in one possible embodiment is a moveable source.
Some operator input may be used in the method. Thus, in another aspect, a method of imaging the interior of the eye of a patient is provided, comprising the steps of: collecting at least one image in a series of images of the interior of the eye with a camera; processing the images to determine regions of the interior of the eye that have been imaged by the camera; and displaying the at least one image to an operator, e.g., on a screen display associated with the camera. The operator provides input as to additional regions of the interior of the eye that are to be imaged by the camera. The camera is operated and/or the patient is instructed to shift their line of sight in a direction so as to image additional regions of the interior of the eye not previously imaged in the series of images.
In another related aspect, a camera is provided for imaging the interior of the eye. The camera includes a digital imaging device (e.g., CCD video camera) adapted for capturing a series of images of the interior of the eye, and a processing system including software for processing the series of images captured by the digital imaging device to determine regions of the interior of the eye that have been captured by the digital imaging device. The camera further includes comparison software comparing regions determined by the processing software to have been imaged by the camera with a predetermined identification of regions of the interior of the eye of the patient that are to be imaged by the camera. The camera further includes feedback means, such as a speaker providing audible prompts or one or more lights that can be selectively activated, or a moving light source. The feedback means is responsive to the comparison software for operating the camera and/or instructing said patient to shift their line of sight so as to image additional regions of the interior of the eye not previously imaged in the series of images.
The invention is particularly well suited for evaluation of patients for eye diseases including diabetic retinopathy, age-related macular degeneration and glaucoma.